Phosphorous acid triesters (organic phosphites) are compounds which are sensitive to oxidation and can easily be hydrolyzed. They are used as antioxidants in lubricants, where they collect oxygen and are oxidized to phosphates. As stabilizers for polyvinyl chloride, their action is based on the fact that, under the influence of air, UV rays, and/or heat, they are capable of bonding the hydrogen chloride liberated from the polymer (compare, for example, British Patent 803,557 and U.S. Pat. No. 3,516,963).
As well as being used as antioxidants in lubricants and stabilizers in organic polymers, organic phosphites are used as co-catalysts in hydroformylation. Rhodium/phosphite catalyst systems are highly active in this reaction, and are especially suitable for olefins which are difficult to hydroformylate, such as n-butene-2, 2-methylpropene, vinyl acetate, cyclohexene, acrolein and acrylonitrile.
Organic phosphites are very sensitive to hydrolysis; they are hydrolyzed with water (more rapidly in the presence of bases, and still more rapidly by aqueous acids) to give phosphorous acid diesters (secondary phosphites), as well as monoesters (primary phosphites) or free phosphorous acid. In the course of time, even traces of water can thus deactivate the Rh/phosphite system employed as the hydroformylation catalyst.
The rate of hydrolysis depends greatly on the nature of the ester radicals. Trimethyl phosphite is the most unstable; as the chain length increases, the phosphites become more stable to hydrolytic influences. According to A. E. Arbusov and M. G. Imaev [C.A. 51, 1374 g (1957], the rate of hydrolysis of triphenyl phosphite lies between that of triethyl phosphite and that of tripropyl phosphite.
M. G. Imaev [C.A. 55 24531 f (1961)] found that the addition of organic and inorganic bases retards the hydrolysis of trialkyl phosphites, and triethylamine has a better action than pyridine. The author assumes that, by formation of a salt, the base collects the secondary phosphite initially formed and in this way delays further hydrolysis.
European Patent 285,136 A2 describes a process for the separation of secondary and tertiary phosphites. For this, water and an amine are added to a solution of the phosphites in an organic solvent. The salt of the primary phosphite is formed from the secondary phosphite and the tertiary phosphite is separated therefrom.
European Patent 149,894 A2 relates to a hydroformylation process in which a rhodium complex compound containing carbon monoxide and, as a further ligand, a cyclic phosphite is employed as the catalyst. A tertiary amine is also added to the catalyst to stabilize the phosphite. This collects acid cleavage products, which are formed by acid hydrolysis of these cyclic phosphites during the hydroformylation and deactivate the catalyst. The cleavage products also catalyze further hydrolysis of the phosphites. According to European Patent 149,894 A2, the ability of tertiary amines to stabilize the cyclic phosphites extends to cyclic phosphites but not to acyclic (open-chain) phosphites.
Y. Matsui [Bulletin of the Japan Petroleum Institute 19, No. 1, pages 62-67 (1977)] employed the system RhH(CO) (PPh.sub.3).sub.3 /P(OPh.sub.3) as a hydroformylation catalyst. The life of this catalyst was only four hours. After addition of tri-n-octylamine, it was possible to increase this life to more than 10 hours. The action of the amines is based on collection of the acid cleavage products of the phosphites. They are reported to stabilize the catalyst by preventing the cleavage products of the phosphites from forming coordination complexes with the rhodium.
European Patents 167,969 A2 and 143,464 B1 describe phosphites with additions of amines which have a greater stability to water than the same phosphites without added amine.
Japanese Patent Application 81/113,790 describes the influence of diethanoldodecylamine on the hydrolysis of distearylpentaerythritol diphosphite. A mixture of 100 parts by weight of phosphite and 5 parts by weight of amine absorbs 7.1% water within 96 hours at 20.degree. C. and 90% relative humidity. Under otherwise identical conditions--but without addition of amine--the phosphite absorbs 21.4% water.